Digi WAN 3G User Manual page 212

TXD
Transmit eXchange Data.
Universal Mobile Telecommunications Service (UMTS)
A third-generation (3G) broadband, packet-based transmission of text, digitized voice, video,
and multimedia at data rates up to 2 megabits per second (Mbps) that offers a consistent set of
services to mobile computer and phone users no matter where they are located in the world.
Based on the Global System for Mobile (GSM) communication standard, UMTS, endorsed by
major standards bodies and manufacturers, is the planned standard for mobile users around the
world and is at present still being made available. Once UMTS is fully available geographically,
computer and phone users can be constantly attached to the Internet as they travel and, as they
roam, have the same set of capabilities no matter where they travel to. Users will have access
through a combination of terrestrial wireless and satellite transmissions. Until UMTS is fully
implemented, users can have multi-mode devices that switch to the currently available
technology (such as GSM 900 and 1800) where UMTS is not yet available.
Today's cellular telephone systems are mainly circuit-switched, with connections always
dependent on circuit availability. A packet-switched connection, using the Internet Protocol (IP),
means that a virtual connection is always available to any other end point in the network. It will
also make it possible to provide new services, such as alternative billing methods (pay-per-bit,
pay-per-session, flat rate, asymmetric bandwidth, and others). The higher bandwidth of UMTS
also promises new services, such as video conferencing. UMTS promises to realize the Virtual
Home Environment (VHE) in which a roaming user can have the same services to which the user
is accustomed when at home or in the office, through a combination of transparent terrestrial and
satellite connections.
The electromagnetic radiation spectrum for UMTS has been identified as frequency bands 1885-
2025 MHz for future IMT-2000 systems, and 1980-2010 MHz and 2170-2200 MHz for the
satellite portion of UMTS systems.
User Datagram Protocol (UDP)
A communications protocol that offers a limited amount of service when messages are
exchanged between computers in a network that uses the Internet Protocol (IP). UDP is an
alternative to the Transmission Control Protocol (TCP) and, together with IP, is sometimes
referred to as UDP/IP. Like the Transmission Control Protocol, UDP uses the Internet Protocol
to actually get a data unit (called a datagram) from one computer to another. Unlike TCP,
however, UDP does not provide the service of dividing a message into packets (datagrams) and
reassembling it at the other end. Specifically, UDP does not provide sequencing of the packets
in which the data arrives, nor does it guarantee delivery of data. This means that the application
program that uses UDP must be able to make sure that the entire message has arrived and is in
the right order. Network applications that want to save processing time because they have very
small data units to exchange (and therefore very little message reassembling to do) may prefer
UDP to TCP. The Trivial File Transfer Protocol (TFTP) uses UDP instead of TCP. UDP
provides two services not provided by the IP layer. It provides port numbers to help distinguish
different user requests and, optionally, a checksum capability to verify that the data arrived
intact. In the Open Systems Interconnection (OSI) communication model, UDP, like TCP, is in
layer 4, the Transport Layer.
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